Previously, III-V materials with a bandgap lower than 1.0 electronvolt (eV) could only be realised on indium phosphide (InP) substrates. However, gallium arsenide (GaAs) substrates are preferred to InP substrates from both a technical and economic perspective. The aluminium gallium arsenide/gallium arsenide (AlGaAs/GaAs) mirror system has higher refractive index difference and better thermal dissipation than the gallium indium arsenide phosphide/indium phosphide (GaInAsP/InP) mirror system. GaAs substrates are also known to be more robust and inexpensive compared to InP substrates. In addition, the small difference in lattice constant between GaAs and germanium (Ge) (˜0.04%) and the recent availability of Ge-terminated Si substrates using graded silicon germanium (SiGe) buffer layers have opened up the possibility of integrating III-V devices on silicon (Si) substrates. These advantages and the potential of GaAs-based processes have fuelled a need for small bandgap materials that can be grown lattice-matched to the GaAs substrate.
In 1992, it was found that incorporation of a small amount of nitrogen (0% to 4%) into GaAs resulted in a reduction of the bandgap and lattice constant in GaAsN. This was unusual at that time as bandgap reduction by addition of other atoms, such as indium or antimony, to GaAs was expected to increase its lattice constant. The reduction of bandgap on addition of nitrogen was explained by the band anti-crossing model, where introduction of nitrogen atoms results in a nitrogen-related state above the conduction band minima. This nitrogen-related state splits the conduction band and reduces the conduction band minima, which results in a reduction of the bandgap energy.
The application of GaNAsSb material for near infrared applications was first proposed in 1999. GaNAsSb can be lattice-matched to GaAs by keeping the ratio of the antimony content to the nitrogen content at approximately 2.6:1. In addition, the bandgap of GaNAsSb can be lowered to the near infrared region by increasing the nitrogen concentration. Antimony was also found to have a surfactant effect in dilute-nitride growth. This surfactant effect encourages the incorporation of nitrogen atoms into substitutional sites and thereby prevents formation of nitrogen related defects. Furthermore, the GaNAsSb material offers flexibility for independent tuning of the conduction band offset and valence band offset. In particular, the conduction band offset can be tuned by varying the nitrogen content, while the valence band offset can be tuned by varying the antimony content. This gives the ability to engineer the bandgap for electronic and optoelectronic device applications.
There are, however, shortcomings in GaNAsSb when prepared using conventional methods. For instance, the nitrogen plasma ignition process damages the surface of the semiconductor prior to growth of GaNAsSb due to unstable plasma conditions at the ignition stage. Energetic bombardment of reactive ion species in the nitrogen plasma onto the GaNAsSb surface during growth also contributes to defect creation. Additionally, due to the presence of arsenic antisite defects and nitrogen related defects, carrier recombination centres are created inside the bandgap. This reduces the carrier lifetime of the GaNAsSb and compromises its luminescence. Furthermore, high unintentional doping (i.e. greater than 1×1017 per cubic centimeter) caused by the presence of nitrogen-related defects, with an energy level of about 0.1 eV above the valence band, limits the application of GaNAsSb for photon absorption. Photon absorption requires a thick depletion region, and a high unintentional doping leads to a counteractive effect. Also, the presence of arsenic antisite defects and nitrogen-related defects promote trap-assisted tunnelling in a reverse-biased GaNAsSb—GaAs p-n junction, leading to high leakage current (i.e. greater than 10 amperes per square centimeter).
In addition, dilute nitride materials (with less than 4% of nitrogen) are known to exhibit poor material quality, resulting in poor device performance.